Since the power sources used by most electronic equipments are DC power sources, it is necessary to use a power supply device or a rectifier to convert AC power into electric power of various DC voltages required by different electronic equipments. Based on different circuit designs, the power supply device can be sorted mainly into two types, linear power supply device and switching power supply device. The switching power supply device comes with a more complicated circuit layout, a larger ripple, and a larger electromagnetic interference than the linear power supply device. However, the switching power supply device has many advantages including high conversion efficiency, low power consumption at empty load, and light weight. Overall speaking, the switching power supply device is still superior to the linear power supply device, and thus becoming a mainstream of power supply devices in the market.
In general, a switching power supply device is operated at high frequency to reduce the size of required electronic components, but the high operation frequency also causes an issue of electromagnetic interference (EMI). In addition to generating the noise to the power supply that will affect the electronic equipments, the electromagnetic wave also affects nearby wireless communication equipments and transmissions of radio and television signals.
Traditionally, an EMI filter is installed at an input terminal of a power supply to reduce electromagnetic interference. The EMI filter is generally composed of an inductor, a capacitor, and a resistor for filtering the electromagnetic interference. However, the larger the electromagnetic interference, the larger the EMI filter is required, and thus incurring a higher cost of the circuit. In addition, the EMI filter cannot handle the radiation due to electromagnetic interference.
With reference to FIG. 1 for a conventional frequency jitter power supply device, a rectified voltage 15 is produced after an AC voltage 5, which is filtered by an EMI filter 120, is rectified by a rectifier 10. The rectified voltage 15 is inputted to a primary winding 35 of a transformer 40 after the rectified voltage 15 is filtered by the filter capacitor 20 so as to generate output on a secondary winding 45 of the transformer 40. The output of the secondary winding 45 is rectified through a secondary rectifier 50 to a capacitor 55 to generate an output voltage 60 at a power output terminal 65. The PWM controller 90 receives a feedback signal from a feedback loop composed of a feedback resistor 80, a Zener diode 75, and an optical coupler 70 at a feedback pin 85, and adjusts a duty cycle of a built-in transistor switch to modulate an electric power inputted to the primary side 35 for the effect of stabilizing the output voltage 60.
A PWM controller 90 receives a jitter current 135 from an electromagnetic interference resistor 140 at frequency jitter pins 125 and 130. The jitter current 135 varied with the change of ripple composition of the rectified voltage 15 changes the frequency of a triangle wave signal from a frequency generator in the PWM controller 90. The triangle wave signal of the frequency generator serves as a comparison reference of the feedback signal, and thus affects the switching frequency of the transistor switch. Thereby, the switching frequency of the transistor switch would be expanded to a wider frequency band, so that the peak of the electromagnetic interference wave can be dropped down to achieve the effect of reducing electromagnetic interference.
Since the ripple composition varies with the input voltage and the output load, it is difficult to select an appropriate electromagnetic interference resistor 140 in practical applications. In addition, the range (or percentage) of frequency jitter also varies with the input voltage, which makes the usage and design more inconvenient and difficult.